Spergualin is a compound isolated from the culture filtrate of a spergualin-producing strain of the genus Bacillus by Umezawa et al., who are also the inventors of this invention. It has the following structure: ##STR2##
Spergualin exhibits a growth inhibitory activity against Gram-positive and -negative bacteria. In therapeutic experiments, it exhibits also a distinctive curing effect and life extension effect on mouse leukemia L-1210 and FL-4, Ehrlich carcinoma, and sarcoma 180 (S-180). So, it is a compound expected to be an antitumor agent (B.P. Publication No. 2,084,999A). It is known that spergualin can be obtained by chemical synthesis {J. Antibiotics, vol. 34, 1625 (1981)}.
It is also known that 15-deoxyspergualin, a deoxy derivative at position 15 of spergualin, has similar effects to those of spergualin. Further, Umezawa et al. conducted a research on 15-O-acylspergualin, which is an acyl derivative of the hydroxyl group at the position 15 of spergualin, and, as a result, found that this compound has also similar biological activities. Although these compounds have a distinguished carcinostatic activity, their insufficient stability in aqueous solution has hindered them from clinical application.
The present inventors further conducted an extensive study to find a spergualin-related compound which is stable in aqueous solution, yet retaining the antibiotic activity. As a result, this invention has been accomplished based on the finding that the above object can be achieved by converting the group ##STR3## at positions 10 to 12 into various amino acid residues.